8344e00e   
   XPost: sci.electronics.design   
   From: siegman@stanford.edu   
      
   > This starts to get a little subtle, I think.  ISTM that you have to draw   
   > a conceptual line between processes that contribute randomness, i.e.   
   > primary photodetection and diffusion, and ones that don't, e.g. the RC   
   > rolloff due to the photodiode capacitance.  So let's model the PD as two   
   > noisy current sources in parallel--a slow one and a fast one--with a   
   > capacitor in parallel.   
   >   
   > I'd be willing to bet that both the fast and slow current sources   
   > produce shot noise equal to sqrt(2*e*I_N), where I_N is the   
   > instantaneous current arriving by the Nth mechanism.   
      
   Of historical interest:  (Two different Anders(s)ons,   
   both papers good)   
      
   =========================================   
      
   Anderson, Larry K. ;   
      
   Measurement of the microwave modulation frequency response of junction   
   photodiodes  [using microwave noise measurements]   
      
   This paper appears in:  Proceedings of the IEEE   
   Issue Date :  May 1963   
   Volume :  51 ,  Issue:5   
   On page(s): 846--847   
      
   =========================================   
      
   Torbjörn Andersson, Alan R. Johnston, and Hans Eklund   
      
   Temporal and frequency response of avalanche photodiodes from noise   
   measurements   
      
   Applied Optics, Vol. 19, Issue 20, pp. 3496-3499 (1980)         
      
   Abstract   
      
   This paper describes a method of obtaining the temporal and frequency   
   response of avalanche photodiodes (APD) by performing simple noise   
   measurements. From the measured noise spectrum and by using the Hilbert   
   transformation technique, the complex transfer function of the detector   
   is determined. response can then easily be calculated by means of fast   
   Fourier transforming. The method has been applied on a high speed APD,   
   with a bandwidth of ~2 GHz, and on a relatively slow APD, with a   
   bandwidth of 0.2 GHz, to calculate the pulse response from a short   
   optical pulse. The calculated pulse width for the fast APD was 215 psec,   
   and the corresponding measured width was 210 psec, while for the slow   
   APD the calculated and the measured widths both were 3.1 nsec. Also the   
   shapes of the pulse responses showed excellent agreement. The method   
   depends on the essentially identical frequency response of an APD and   
   associated circuits for noise due to steady-state illumination and for a   
   signal.   
      
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